A method and appertaining system load 3D shape information defining a hearing aid shell into a processor and present a representation of the shell on a display. A fill boundary is entered by a user operating a user input device and a fill region for the shell is thus defined. The fill boundary is then displayed, but can be modified by the user. Various checks may be performed to ensure that the fill region is a proper one, and if not, a status can be provided to the user indicating why the fill region is unacceptable. The displayed shell can be rotated and moved to assist the user. Once an acceptable fill region has been defined, an indication is provided by the user that the displayed fill region is to be used as the actual fill region.